1. Field of the Invention
This invention relates to electrochromic devices.
2. Art Background
Electrochromics have potentially significant advantages over other display devices, and therefore, are presently being contemplated for use in large area displays. For example, the electrochemical reactions relied on in electrochromic devices usually require only small amounts of d.c. power. The reduced power involved in operating electrochromic devices limits the difficulties involved in contemplated circuitry design for large scale arrays. Additionally, since electrochromics rely on electrically induced chemical changes, such devices usually have a memory. That is, once a chemical change is initiated by an appropriate electrical signal, the induced chemical state, with a different coloration than the initial state, is typically very stable. Within a reasonable time period, the color does not change until a suitable electrical impulse is supplied which reverses the chemical reaction. This memory property is essential for many applications such as indicator displays.
The stability provided by an electrically induced chemical reaction, at least a priori, also offers the possibility of long term reliability. However, secondary effects often defeat this inherent long term stability. For example, tungsten bronzes are one type device which utilize a solid electrochromic in contact with an electrolyte. An electrode containing material such as H.sub.x WO.sub.3 or Li.sub.x WO.sub.3 (0&lt;x&lt;1) is contacted with an electrolyte such as sulfuric acid. These cells initially exhibit the desirable properties, such as memory, inherent in electrochromics although the response times are too long for many applications. However, a significant degradation of properties due to corrosion--a common disadvantageous secondary effect--is prevalent in such devices. Although the cells have usable response times for some applications, continued use produces a total failure of the tungsten bronze electrode. (See for example, Randin, J. P. Journal Electronic Materials, 7, 47 (1978).)
It is generally postulated that this unacceptable phenomenon results from the dissolution of the tungsten bronze in the electrolytes used. Attempts to substitute a less corrosive medium while maintaining response times have not been successful. (See, for example S. K. Mohapatra, J. Electrochem. Soc. 125, 284 (1978).) Thus, the inherent attributes of electrochromic devices are alluring. However, problems such as corrosion and excessive response times are significant in these devices.